diff --git a/tests/domain/selection.test.ts b/tests/domain/selection.test.ts
new file mode 100644
index 00000000..6a15fdba
--- /dev/null
+++ b/tests/domain/selection.test.ts
@@ -0,0 +1,104 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  applyEditorSelectionClick,
+  getSelectionDefaultActiveId,
+  resolveSelectionActiveId
+} from "../../src/core/selection";
+
+describe("selection click helpers", () => {
+  it("toggles same-kind multi-selection membership and keeps the last-added item active", () => {
+    const firstSelection = applyEditorSelectionClick(
+      {
+        kind: "none"
+      },
+      {
+        kind: "brushes",
+        ids: ["brush-a"]
+      },
+      false
+    );
+    const secondSelection = applyEditorSelectionClick(
+      firstSelection,
+      {
+        kind: "brushes",
+        ids: ["brush-b"]
+      },
+      true
+    );
+
+    expect(secondSelection).toEqual({
+      kind: "brushes",
+      ids: ["brush-a", "brush-b"]
+    });
+    expect(getSelectionDefaultActiveId(secondSelection)).toBe("brush-b");
+
+    const thirdSelection = applyEditorSelectionClick(
+      secondSelection,
+      {
+        kind: "brushes",
+        ids: ["brush-c"]
+      },
+      true
+    );
+
+    expect(thirdSelection).toEqual({
+      kind: "brushes",
+      ids: ["brush-a", "brush-b", "brush-c"]
+    });
+    expect(getSelectionDefaultActiveId(thirdSelection)).toBe("brush-c");
+
+    const removedActiveSelection = applyEditorSelectionClick(
+      thirdSelection,
+      {
+        kind: "brushes",
+        ids: ["brush-c"]
+      },
+      true
+    );
+
+    expect(removedActiveSelection).toEqual({
+      kind: "brushes",
+      ids: ["brush-a", "brush-b"]
+    });
+    expect(resolveSelectionActiveId(removedActiveSelection, null)).toBe(
+      "brush-b"
+    );
+  });
+
+  it("replaces the selection when shift-clicking a different selectable kind", () => {
+    const nextSelection = applyEditorSelectionClick(
+      {
+        kind: "brushes",
+        ids: ["brush-a", "brush-b"]
+      },
+      {
+        kind: "entities",
+        ids: ["entity-main"]
+      },
+      true
+    );
+
+    expect(nextSelection).toEqual({
+      kind: "entities",
+      ids: ["entity-main"]
+    });
+    expect(getSelectionDefaultActiveId(nextSelection)).toBe("entity-main");
+  });
+
+  it("clears on empty click without shift and preserves on empty shift-click", () => {
+    const currentSelection = {
+      kind: "modelInstances" as const,
+      ids: ["model-a", "model-b"]
+    };
+
+    expect(
+      applyEditorSelectionClick(currentSelection, null, false)
+    ).toEqual({
+      kind: "none"
+    });
+    expect(applyEditorSelectionClick(currentSelection, null, true)).toEqual(
+      currentSelection
+    );
+  });
+});